Non-invasive techniques in medical diagnoses have expanded dramatically in the past 10 years. Nuclear magnetic resonance of hydrogen has already proven to be well suited for the depiction of anatomy with excellent spatial resolution. However, this technique suffers from long acquisition times and is not amenable to obtaining metabolic information.
Positron emission tomography (PET), on the other hand, has the ability to use radiolabeled natural metabolic substrates in tracer concentrations to assess basic metabolic activity. However, this technique possesses poor spatial resolution and requires an on-site cyclotron. PET relies on ionizing radiation for its operation.
Fluorine (.sup.19 F) NMR/MRI shows promise but also poses problems in its initial stages of development toward practical application. Fluorine-19 has a high NMR sensitivity (about 83% that of hydrogen), negligible biological background, a resonance frequency only 6% lower than hydrogen, a spin of 1/2, and 100% natural abundance. Hydrogen NMR/MRI equipment may also be used for detection of fluorine-19.
Commercially available fluorinated compounds used for MRI or NMR studies generate a wide range of resonance frequencies resulting from differences in local electron environments affecting the fluorine nucleus. Several dozen compounds have been used in vivo studies either as inert agents or metabolic agents. The inert agents include perfluorinated compounds such as perfluoro-tri-n-butyl amine, Fluosol.RTM. perfluorocarbon mixture (a mixture of perfluorotripropylamine and perfluorodecalin) (distributed by Alpha Therapeutic Corp., Los Angeles, Calif. for Green Cross Corp., Osaka, Japan) and perfluorooctyl bromide and fluoromethanes and fluoroethanes such as trifluoromethane, chlorodifluoromethane and halothane. Metabolic agents include monofluorinated aliphatics such as 5-fluorouracil and fluorodeoxy glucose and fluorinated aromatics, such as trifluoromethyl aromatics and monofluoro aromatics.
These compounds fail to provide fluorine in sufficient numbers (for example, 5-fluorouracil and 3-fluoro-3-deoxy-D-glucose) or may contain broad multiple peaks (for example, perfluorodecalin and perfluorotri-n-butylamine) both being detrimental to imaging. The combination of chemical shift effects and decreased intensity of split signals (multiple peaks) reduces the detectability of any signal, thereby requiring longer imaging times. Present animal studies require greater than one hour acquisition time for fluorine-19 MRI studies even at high concentrations of the fluorinated compound utilized, for example, 2.4M in rabbits of standard size, representing 30% blood replacement. D. Eidelberg et al., ".sup.19 F NMR Imaging of Blood Oxygenation in the Brain," Magnetic Resonance in Medicine, vol. 6. pages 344-352 (1988). This lengthy process could be shortened and concentration reduced if a stronger signal emanated from the sample.
Though perfluorination of biological compounds would increase the number of fluorines therein, the chemical and biological behavior of such perfluorinated compounds, particularly metabolic compounds, would likely not be equivalent to their non-fluorinated condition, i.e., natural compound. Further, such perfluorinated compounds would suffer from decreased intensity of split signals.
There has been much interest in the use of PFCs as oxygen carrying agents. In addition to their use as blood substitutes, experimental work has shown their efficiency in delivering oxygen to ischemic tissue and hence their potential as therapeutic agents in the treatment of cerebral and myocardinal ischemia. These compounds have also found application in NMR vascular imaging. PFCs contain a high concentration of .sup.19 F atoms and can be imaged with machines designed for hydrogen protons with only minor modifications in tuning. As .sup.19 F is virtually absent from biological tissues, intravenously infused PFC emulsions are an excellent vascular marker and may be used to image the perfusion of tissues of high vascularity. There are, however, a number of problems which limit the signal-to-noise ration (SNR) of PFC images. First, the concentration of .sup.19 F is low relative to .sup.1 H concentration (around 2.4M in blood after 30% volume replacement compared with a .sup.1 H concentration of 80M). Second, infused PFCs have short transverse relaxation times (T.sub.2 's) and therefore the NMR signal decays rapidly after it has been produced. In addition PFCs have a spectrum with several peaks at different resonant frequencies giving rise to the misregistered superimposition of images from each peak. Eliminating this chemical-shift artifact often involves loss of the signal from the suppressed portion of the spectrum. Nonetheless, because of the relatively high intrinsic vascularity of the mammalian cerebral cortex D. Eidelberg et al. thought that in-vivo .sup.19 F brain imaging with infused PFC might be feasible.
PFCs have the added advantage of potential use as indicators of intravascular oxygenation. Molecular oxygen is paramagnetic and enhances spin-lattice relaxation by dipole-dipole interactions in such a way that the rate 1/T, increases linearly with .sub.p O.sub.2 (6). Pairs of .sup.19 F partial saturation spin-echo (PSSE) images of varying repetition time (T.sub.R) may be used to compute a T.sub.1 map and the 1/T.sub.1 vs .sub.p O.sub.2 calibration line may then be applied to calculate .sub.p O.sub.2 in vessels (8). A limitation of PFCs is the multi-resonant signals resulting from non-equivalent fluorine nuclei. Use of inert compounds with 2 to 4 reporter groups, containing 18 to 36 fluorine atoms would greatly enhance the sensitivity of measurement of regional O.sub.2 concentration. Such a technique made possible in humans would clearly have important ramifications in all organ systems.
Therefore, there exists a need to provide a fluorine-derivatized compound for introducing fluorine to a site or process specific biological compound which would not only allow visualization of a given organ, but also provide metabolic data without suffering the aforementioned disadvantages of known compounds when NMR or MRI studies are performed using same.